Since an inorganic glass has excellent general properties such as excellent transparency and the like, the inorganic glass has been widely used in many fields as an optical member. However, the inorganic glass has drawbacks such that it is heavy and easily broken and low workability and low productivity. As a result, a transparent optical resin has been actively under development as a material for replacing the inorganic glass.
An amorphous thermoplastic resin having typical examples of an acrylic based resin, a styrene based resin, a polycarbonate resin, a polyester based resin, an olefin based resin, an alicyclic acrylic based resin, an alicyclic olefin based resin, a polyurethane resin, a polyether resin, a polyamide resin and a polyimide resin, or a curable resin such as an epoxy resin, an unsaturated polyester resin or a silicon resin has excellent transparency in the visible wavelength region. Besides, such a resin is a general-purpose transparent resin material having excellent properties such as moldability, mass productivity, flexibility, toughness, impact resistance or the like as compared to the inorganic glass material.
It has been expected that materials of high refractive index optical members such as a thin and lightweight optical lens (a spectacle lens, a Fresnel lens, a pickup lens in information recording devices such as CD, DVD and the like, a lens for cameras such as a digital cameras and the like), an optical prism, an optical waveguide, an optical fiber, a thin-film molded product, an adhesive for optical uses, a sealing material for optical semiconductors, a diffraction grating, a light guiding plate, a liquid crystal substrate, a light reflection plate, an anti-reflection plate and the like are developed by providing such a transparent resin material with a high refractive index.
For example, in the field of spectacle lenses, in order to satisfy the need of fashionability, the center thickness, edge thickness and curvature of a lens are required to be reduced to make the lens generally thin. From this point, ever higher refractive index has been in demand.
In recent years, high refractive index has been actively studied by using a monomer containing an element having a large atomic number such as sulfur, halogen or the like. Examples thereof include a resin (nd of from about 1.60 to 1.67) obtained by subjecting a thiol compound and an isocyanate compound to thermal polymerization to form a thiourethane bond, a resin (nd of about 1.7) obtained by subjecting an episulfide or epithiosulfide compound to polymerization/curing and the like.
Similarly, there has been strongly demanded a general-purpose transparent resin having a high refractive index, particularly a high refractive index resin having a refractive index of 1.7 or more such as an amorphous thermoplastic resin having typical examples of an acrylic based resin, a styrene based resin, a polycarbonate resin, a polyester resin, an olefin based resin, an alicyclic acrylic resin, an alicyclic olefin resin, a polyurethane resin, a polyether resin, a polyamide resin and a polyimide resin, and a curable resin such as an epoxy resin, an unsaturated polyester resin, a silicon resin or the like.
The refractive index of an organic resin is determined depending on the element in use and molecular structure so that it is also increased by introduction of such a halogen element or a sulfur element. However, the refractive index is usually limited to the range of about 1.4 to 1.7.
For example, in a polymer optical fiber using an acrylic based resin such as polymethyl methacrylate (PMMA) or the like, a core portion (central portion in the cross section of the optical fiber) has higher refractive index than that of a clad portion (outer peripheral portion). It is possible to increase the numerical aperture corresponding to the maximum angle capable of propagating light as the difference of refractive indexes is greater.
For example, in a light emitting diode, a light emitting element portion is sealed with an epoxy resin or the like. In general, when the refractive index of a semiconductor constituting a semiconductor element portion is extremely high and the refractive index of a material in contact therewith is low, a critical angle is also small and total reflection easily occurs. Accordingly, a light emitting element is wrapped with a material having a higher refractive index, whereby it is possible to increase the angle causing total reflection. Thereby, the luminous-flux extraction efficiency on the outside of the portion is improved.
Furthermore, there has been demanded that a component comprised of a plurality of materials having different refractive indexes, for example, an optical fiber, an optical waveguide and a part of a lens, should be developed, and materials having a refractive index distribution should be developed as well. In order to cope with the demand of these materials, it is essential that the refractive index can be freely controlled.
Such a resin to be used for an optical component, for example, a thermoplastic resin having typical examples of an acrylic based resin, a styrene based resin, a polycarbonate resin, a polyester resin, an olefin based resin, an alicyclic acrylic resin, an alicyclic olefin resin, a polyurethane resin, a polyether resin, a polyamide resin, a polyimide resin and the like; a curable resin such as an epoxy resin, an unsaturated polyester resin, a silicon resin or the like; or a resin obtained by polymerizing a monomer containing an element having a large atomic number such as sulfur, halogen or the like is strongly demanded to have a high refractive index.
In recent years, for the purpose of achieving a resin having a high refractive index, there has been proposed a technique for forming a colorless and transparent resin having a high refractive index by introducing transparent inorganic oxide fine particles with a high refractive index having a crystal structure such as Zr, Sn, Sb, Mo, In, Zn, Ti or the like or complex oxides thereof into the resin while maintaining the dispersion state (Patent Document 1, Non-Patent Document 1 and the like). However, in the above technique, in order to maintain high dispersibility and transparency, it needs to introduce a sulfonic acid group into the resin, thus leading to deterioration of physical properties such that the hygroscopicity becomes large or the like.
On the other hand, in order to suppress the cohesive force and surface activity of particles in the resin, there have been proposed metal oxide fine particles having a surface coating layer containing an organic material (Patent Documents 2 and 3). However, in these techniques, it has not yet been sufficient enough to design a resin having a high refractive index while using the amount of a matrix capable of maintaining strength or the like. Namely, when metal oxide fine particles are excessively added to improve the refractive index, the resin becomes, on the contrary, fragile.
As described above, there has been demanded a composite material which is transparent without causing agglomeration even though fine particles are incorporated in a high proportion, and further has a high refraction index and high strength in various product fields.
Patent Document 1: Japanese Patent Laid-open No. 2007-246334
Patent Document 2: Japanese Patent Laid-open No. 2007-204739
Patent Document 3: Japanese Patent Laid-open No. 2007-238661
Patent Document 4: Japanese Patent Laid-open No. 2007-270097
Patent Document 5: Japanese Patent Laid-open No. 2005-194148
Non-Patent Document 1: Polymer Preprints, Japan, Vol. 56, No. 2 (2007) pp. 3047-3048